Bar tacking is the term used to describe the sewing of small stitch patterns which are generally used for reinforcing joints in shoes and other garments. These patterns are generally limited to a specific number of stitches in the range of from 10 to 100 stitches per pattern and cover only a small area of the workpiece. The operation is performed by moving the workpiece under the needle and this motion is achieved automatically by means of a work clamp which is mounted for movement along two axes relative to the needle. Work clamp movement is controlled by a style or feed cam which is operatively linked to the clamp. The style cam is generally driven by means of a shaft connected to the main needle bar drive shaft through a gear train. Thread cutting is controlled by a second can connected to the same shaft but mounted opposite to the style cam. In this manner a limited amount of automatic operation is achieved. However, the variety of patterns are limited by cam design and the gear ratio between the needle drive shaft and the cam shaft since each pattern must be completed within one rotation of the cam. This necessitates the replacement of the style cam for each change in pattern and in addition, if the number of stitches in the pattern changes, a new gear train must be installed. This may require anywhere from two to as long as eight hours effort by a skilled mechanic and results in a significant loss of production per machine.
The purpose of this invention, therefore, is to provide an automatic bar tacker which is free of the restrictions of the style cam, thereby eliminating the need for costly changes resulting in loss of production. This is achieved by the replacement of the style cam with a numerically controlled drive. The mechanics of this drive are described in co-pending U.S. Pat. application Ser. No. 530,048 filed Dec. 5, 1974.
In order to convert the standard bar tacker sewing machine to numerical control, the style and knife cams are removed from their shafft. The knife cam is replaced by a new cam which is operatively connected to its shaft through an electrically operated clutch. The new cam is constructed with additional notches to provide collateral functions such as nipper operation and knife positioning. The vertical operating levers formerly engaging the style cam are fitted with gear sectors and each is operatively connected through a pinion to a stepping motor which is mounted on the sewing machine housing. The stepping motors therefore directly replace the style cam. The stepping motors are controlled by a numerical control system which can be adopted to generate signals to cause movement of the workpiece clamp through the desired tack design and to operate the knife cam clutch and other collateral machine functions.